An FMCW (Frequency Modulated Continuous Waves) radar, which is capable of simultaneously measuring the distance and relative speed between the radar and a target object at high accuracy is widely used as an onboard radar. The system transmits a chirp signal as a continuous wave, receives a reflected wave from the target object, and measures the distance to the target object from the delay time of the reflected wave and the speed relative to the target object from a frequency shift. Here, the chirp signal refers to a signal subjected to the frequency modulation which varies the frequency with time.
As a signal generating circuit in an FMCW radar device, for example, a signal generating circuit of a Patent Literature 1 is known. The signal generating circuit of the Patent Literature 1 comprises a modulation controller, a digital to analog converter (DAC), a low pass filter (LPF), a voltage controlled oscillator (VCO), a local signal generator, a difference frequency signal generator, an IF detector and an ADC (Analog to Digital Converter). The modulation controller comprises a lookup table (LUT) that records voltage-frequency characteristics (V-F characteristics) of the VCO.
Next, the operation of the signal generating circuit at the time of generating the chirp signal will be described. The modulation controller obtains from the V-F characteristics of the LUT a control voltage that will linearize the time-frequency characteristics of the chirp signal, and outputs the control voltage to the DAC as a digital signal.
The DAC converts the digital control voltage fed from the modulation controller into an analog control voltage, and outputs the analog control voltage to the LPF.
The LPF eliminates a high-frequency component of the control voltage fed from the DAC, and smoothes the control voltage. Then the LPF outputs the control voltage to the VCO.
The VCO, on the basis of the V-F characteristics it possesses, outputs a signal with the frequency corresponding to the control voltage in accordance with the control voltage fed from the LPF.
As a result, the signal generating circuit generates the control voltage of the VCO on the basis of the V-F characteristics of the VCO, which the LUT possesses, and can generate the chirp signal subjected to the frequency modulation.
Next, the operation of the signal generating circuit at the time of updating the LUT will be described.
The VCO generates the chirp signal (f1) in accordance with the control voltage.
The local signal generator generates the local signal (f2).
The difference frequency signal generator outputs the IF (Intermediate Frequency) signal (f1−f2), which is a difference frequency component between the output signal of the VCO and the local signal, from the output signal (f1) of the VCO and the local signal (f2) generated by the local signal generator.
The IF detector outputs the IF detection signal to the ADC when the frequency of the IF signal becomes not greater than a specified IF detection frequency. That the frequency of the IF signal becomes not greater than the specified IF detection frequency occurs when f1 is nearly equal to f2. Since the frequency f1 varies with time, the timing at which f1 is nearly equal to f2 occurs.
The ADC measures the control voltage (v1) of the VCO at the timing when the IF detector outputs the IF detection signal, and outputs it to the modulation controller. This operation is carried out several times while varying the local frequency f2.
From the variation of the control voltage v1 while varying the local frequency f2, the modulation controller obtains the V-F characteristics of the VCO, and updates the V-F characteristics stored in the LUT. On the basis of the updated V-F characteristics, the modulation controller obtains the control voltage that will linearize the time-frequency characteristics of the chirp signal, and outputs the control voltage to the DAC as the digital signal.
Thus, the signal generating circuit of Patent Literature 1 updates the V-F characteristics stored in the LUT using the local signal generator, difference frequency signal generator, and IF detector, and generates the chirp signal on the basis of the updated V-F characteristics.
However, the circuit of Patent Literature 1 cannot compensate for the error of the chirp signal when the V-F characteristics of the VCO suddenly vary owing to the disturbance of the temperature or the like. The circuit must generate the chirp signal a plurality of times while varying the local frequency f2 to obtain the V-F characteristics of the VCO, and cannot update the V-F characteristics of the VCO during that time. In other words, since the circuit of Patent Literature 1 can detect from a single chirp signal only one voltage value (V) and only one frequency value (F) corresponding to the voltage, it cannot obtain the V-F characteristics of the VCO unless it generates the chirp signal a plurality of times.